JPH0999908A - Shrink packaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To seal without leaving pinholes and prevent deterioration of appearance of a sealing part at a side part due to deviation to a top face or pressed trace, in a shrink packing of a tray having contents housed therein. SOLUTION: A plastic film having a heat shrink of 20% or above both vertically and laterally, an average heat shrink in vertical and lateral directions of 35% or higher, and an average tensile work recovery ratio after heat shrinkage of 30% or higher is used for packing a tray. The inside of it is deaerated and an opening is sealed. A plastic film at the bottom face of the tray is shrunk with heat and the sealed part at the side part is pulled downward. Thereafter, the plastic film on the top face of the tray is shrunk with heat.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a tray containing contents such as foods, which is wrapped with a heat-shrinkable plastic film and then heat-shrinked to adhere the contents and the tray. Regarding shrink packaging.

[0002]

2. Description of the Related Art As is often seen in supermarkets, convenience stores and the like, shrink wrapping is widely known in which a tray containing contents such as fresh food is wrapped in a heat-shrinkable plastic film. In this shrink wrapping, the tray containing the contents is wrapped in a heat-shrinkable plastic film, the opening is hermetically sealed, a pin hole for deaeration is opened in the plastic film, and the plastic film is passed through a hot air tunnel. It is performed by heat-shrinking and bringing it into close contact with the contents and the tray. In this shrink wrapping, the case where the seals are formed on the two sides (front and back) and the bottom side of the tray (pillow wrapping method) and the case where the seals are formed on the three sides of the tray (half-fold film) Method) and the case where the hermetically sealed parts are formed on the four sides of the tray (upper and lower film method).

[0003]

By the way, in the conventional shrink wrapping described above, it is necessary to open a pin hole in order to allow the internal air to escape when the plastic film is thermally shrunk. In particular, as a food packaging that is not susceptible to invasion of various bacteria, it is desirable that the food can be packaged without opening the pinholes, but there is a problem that pinholes are unavoidable due to the above-mentioned necessity and difficulty in closing afterwards.

Further, in any of the pillow packaging method, the half-folded film method, and the upper and lower film method, there is a hermetically sealed portion on the side of the tray, which is located on the side of the tray when the plastic film is heat-shrinked. A part or the whole of the hermetically sealed portion may slip up to the upper surface side. The upward movement of the hermetically sealed portion deteriorates the appearance of the package and reduces the commercial value and the display effect.

Furthermore, when the plastic film on the tray upper surface is pushed by stacking the packages or picking up the packages by hand, pressing marks are apt to be left as wrinkles and looseness of the plastic films, and the appearance of the package also deteriorates. Therefore, there is a problem that the product value and the display effect are reduced.

The present invention solves the above-mentioned problems of the prior art, and in the shrink packaging of the tray containing the contents, it can be sealed without leaving pinholes, and the side hermetically sealed portion is The purpose of the invention is to prevent the deterioration of the appearance due to the slip-up to the upper surface side and the pressing trace.

[0007]

To this end, in the present invention,
A heat-shrinkable biaxially stretched plastic film having a heat shrinkage of 20% or more in both the length and width, an average value of the heat shrinkage of 35% or more in the length and width, and an average tensile work recovery rate of 30% or more after heat shrinkage. Using the above, the tray containing the contents is wrapped to degas the inside and the opening is hermetically sealed to form a pre-packed body having a hermetically sealed portion on the side of the tray. The shrink-wrapping is performed by heat-shrinking the plastic film on the lower surface side of the tray to pull the side hermetically sealed portion downward and then heat-shrinking the plastic film on the upper surface side of the tray.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The plastic film used in the present invention is a heat-shrinkable biaxially stretched film having a heat shrinkage ratio of 20% or more in both the longitudinal and transverse directions and an average value of the heat shrinkage ratios of the longitudinal and transverse directions of 35% or more. It is a plastic film. If the heat shrinkage ratio is less than 20% in either the lengthwise direction or the widthwise direction, the heat shrinkage at the corners of the tray is insufficient, and the appearance of the obtained package is likely to deteriorate. In addition, when wrapped with a plastic film, which will be described later, it becomes necessary to perform a hermetic seal at a position extremely close to the tray, which impairs practicality in terms of packaging conditions. The same applies when the average value of the heat shrinkage ratios in the vertical and horizontal directions is less than 35%. If both the heat shrinkage ratios in the vertical and horizontal directions and their average values do not meet the above values, a hermetic seal on the side of the tray is used. The parts may slip up onto the upper surface of the tray, or wrinkles and local slack may occur, which significantly impairs the appearance of the package.
Further, it becomes difficult to obtain the recoverability against the indentation deformation described later.

The heat shrinkage is preferably 20% or more in both the vertical and horizontal directions and 40% or more in the average value in the vertical and horizontal directions, and more preferably 25% or more in the vertical and horizontal directions and 45 in the average value in the vertical and horizontal directions.
% Or more. Further, there is no particular limitation on the upper limit of the heat shrinkage rate, but in order to avoid deformation of the general tray during heat shrinkage, it is preferable that both the vertical and horizontal heat shrinkage rates are 90% or less, It is more preferably 85% or less.

The heat shrinkage rate in the present invention is 100.
A mm square plastic film sample was placed in an air oven type thermostatic chamber set to a predetermined temperature and treated for 10 minutes while being able to freely heat shrink, and then the heat shrinkage amount of the plastic film in the vertical and horizontal directions was calculated. A value expressed as a percentage of a value divided by a dimension.

Further, the plastic film used in the present invention has an excellent deformation recovery force when pressed, and in order to prevent pressing marks such as wrinkles and slacks from remaining, the average tensile work recovery rate after heat shrinkage is It is necessary to be 30% or more, preferably 33% or more, and more preferably 35% or more. If the average tensile work recovery rate is less than 30%, the pressing traces are likely to remain and the appearance of the package tends to deteriorate.

The average tensile work recovery rate after the heat shrinkage is a value obtained as follows.

To a square wooden frame having internal dimensions of 240 mm each in the vertical and horizontal directions, a plastic film having a length of 11% in each of the internal dimensions is secured with double-sided tape or the like, and hot air is blown. It is passed through a heating type shrink tunnel (“MS8452.2” manufactured by Kondo Seisakusho) for 5.5 seconds.
The temperature of the hot air at this time is a temperature at which the average thermal shrinkage ratio of the length and width of the plastic film becomes 35%.

The heat-shrinkable plastic film is left to cool at room temperature (usually 18 to 25 ° C.) for about 5 minutes, then removed from the wooden frame, and 10 mm wide and 200 mm long from the center of the plastic film in the vertical and horizontal directions. Cut out a strip-shaped test piece.

The above test piece was attached to a tensile tester adjusted to a chuck distance of 150 mm, and the test piece was pulled at a speed of 200 mm / min until the chuck distance became 195 mm. After returning, the tensile strength of the test piece at that time is recorded as a hysteresis curve. An example of this hysteresis curve is shown in FIG. Area ABCA and area ABOA of the obtained hysteresis curve
Then, the vertical and horizontal tensile work recovery rates are calculated by the following equation,
An average tensile work recovery rate is obtained as an average value of these two values.

Tensile work recovery rate (%) = (area ABCA /
Area ABOA) x 100

Theoretically, the larger the average work recovery rate is, the more preferable, and if it is 100%, the rubber has complete rubber elasticity and can completely prevent the generation of pressing marks. However, achieving this with heat-shrinkable plastic films is practically difficult. Therefore, in the present invention, it is preferable that the average work recovery rate is as large as possible within a realistic range.

The synthetic resin constituting the plastic film used in the present invention is not particularly limited as long as it is a thermoplastic resin. For example, high pressure low density polyethylene, high density polyethylene, ethylene-α-olefin copolymer [linear Low-density polyethylene, ultra-low-density (known as VL, UL) polyethylene, and soft resins with a crystallinity of 30% or less by X-ray method other than those on the left, such as conventional Ziegler-based catalysts. In addition to those obtained by using a catalyst, those obtained by polymerizing with a catalyst commonly known as a single-site catalyst are included], polypropylene-based resins (at least one α-olefin such as ethylene and butene as homo- and comonopolymers) Copolymers), polybutene resins (homopolymers and copolymers with at least one α-olefin such as ethylene or propylene) ), Poly (4-methylpentene) resin, crystalline 1,2 polybutadiene, a soft polymer composed of an α-olefin copolymer different from the ethylene-α-olefin copolymer, an ethylene-vinyl acetate copolymer, an ethylene-aliphatic Unsaturated carboxylic acid copolymer, ethylene-aliphatic unsaturated carboxylic acid ester copolymer, block copolymer of vinyl aromatic hydrocarbon and conjugated diene and at least a part of its double bond are hydrogenated, etc. , Ethylene-vinyl alcohol copolymer, polyamide-based resin, polyester-based resin, and modified polymers (acid-modified, ionized, etc.) of these, and one or two resins selected from these. A blended resin obtained by mixing the above resins or a resin other than the above, which is mainly composed of these resins (meaning that it contains 50% by weight or more) Blend resins can be used.

The plastic film used in the present invention may be a single layer or a multi-layer, but in order to obtain further stability of the hermetically sealed portion, it is preferably a multi-layer film having a heat seal layer, and the layer constitution is symmetrical. Or it may be asymmetric. The resin used for the heat-sealing layer is different from the above resins in the high-pressure low-density polyethylene, ethylene-α-olefin copolymer, polypropylene resin, polybutene-based resin, and ethylene-α-olefin copolymer. α
Soft polymers consisting of olefin copolymers, ethylene-vinyl acetate copolymers, ethylene-unsaturated unsaturated carboxylic acid copolymers, ethylene-unsaturated unsaturated carboxylic acid ester copolymers and the like are preferred, among these It can be used as a selected one kind of resin or a blended resin in which two or more kinds of resins are mixed, or a blended resin in which these resins are mainly contained (meaning that 50% by weight or more is included) and other resins are mixed.

The plastic film used in the present invention contains an antifogging agent, a plasticizer, an antioxidant, an ultraviolet absorber, a lubricant, a colorant, a surfactant, an antiblocking agent, an inorganic filler, a tackifier and the like. It is also possible to perform corona treatment, plasma treatment, and various coating treatments.

The thickness of the plastic film used in the present invention is not particularly limited, but it is 5 to 30 μm from the viewpoint of easily obtaining the average tensile work recovery rate within the above range and from the viewpoint of resource saving and waste reduction. Is preferred,
More preferably 5 to 25 μm, even more preferably 10 to 2
0 μm, optimally 10 to 15 μm. The manufacturing method is not particularly limited as long as it is a biaxial stretching method, and for example, a tenter method, an inflation method (including a double bubble method)
What was manufactured by etc. can be used. The stretching ratio may be the same as or different from the length and the width. Usually, the area stretching ratio is preferably 3 to 50 times, and more preferably 4 to 40 times.

The plastic film used in the present invention is
It may have been subjected to a crosslinking treatment before or after stretching. Further, if necessary, it may be heat-set or laminated with another kind of film for the purpose of ensuring dimensional stability.

In the present invention, the above-mentioned plastic film is used to wrap a tray containing contents such as fresh food, deaerate the inside, and hermetically seal the opening to form a preliminary package.

As the tray, for example, polystyrene foam tray, pulp mold tray, paper tray, polystyrene sheet molding tray and the like can be used. In addition, although the content is not particularly limited, it is generally foods,
In particular, the present invention is suitable for fresh food.

As shown in FIG. 2, the tray containing the contents is wrapped by forming a sealing seal 2 on the two sides (front and rear) of the tray 1 and on the lower surface side, and using a plastic film 3 as a pillow. Pillow wrapping method wrapped in a package, Fig. 3
As shown in FIG. 3, the tray 1 is sandwiched between the half-folded plastic films 3 and the hermetically sealed parts 2 are provided on the three sides of the tray 1.
Of the half-folded film method, and further, as shown in FIG. 4, by sandwiching between the upper and lower two plastic films 3 and 3, the upper and lower film method of forming the hermetically sealed portions 2 on the four sides of the tray 1. Either is fine.

The plastic film can be hermetically sealed by, for example, heat sealing, impulse sealing, ultrasonic sealing or the like.

The deaeration of the inside is performed before all the openings are hermetically sealed, and prevents the package from expanding due to the air inside when the plastic film is heat-shrinked. It is for. Then, by performing this deaeration, it is possible to obtain a package in which the plastic film is sufficiently adhered to the contents and the tray without opening a pinhole.

As a method of degassing, a plastic film is pressed against the contents and the tray with a soft material such as sponge just before the final one-side hermetically sealed portion is formed, and the air inside is removed. Although it can be carried out by pushing out, it is preferable to suck with a vacuum pump in order to ensure deaeration. Immediately after this deaeration, the remaining opening is hermetically sealed to obtain a prepackaged body whose inside is deaerated and hermetically sealed.

When heat-shrinking the plastic film of the above-mentioned prepack, the plastic film on the lower surface side of the tray is first heat-shrinked. For example, hot air, heating steam, infrared rays, hot water, etc. (below "hot air etc.")
It can be done by exposing to. The heat-shrink of the plastic film on the lower surface side of this tray is performed by pulling the sealing seal portion located on the side of the tray downward, and then performing the heat-shrink of the plastic film on the upper surface side of the tray to be performed next. This is to prevent the portion from slipping up to the upper surface side.

In the present invention, after the plastic film on the lower surface side of the tray is heat-shrinked, the plastic film on the upper surface side of the tray is heat-shrinked. The heat shrinkage of the plastic film on the upper surface side of the tray may be performed by applying hot air or the like only from above the tray, but may be performed by applying hot air or the like from all directions.

The temperature of the hot air, etc. applied during the heat shrinkage of the plastic film on the upper surface of the tray is the average temperature at which the maximum heat shrinkage stress of the plastic film appears.
It is preferably 20 ° C. or higher and + 50 ° C. or lower. At a temperature lower than this, it becomes difficult to obtain sufficient deformation recovery, and at a temperature higher than this, the shrinkage stress of the plastic film disappears and it becomes difficult to obtain sufficient deformation recovery.

It is preferable that the temperature of the hot air or the like applied to the plastic film on the lower surface side of the tray during heat shrinkage is set higher than the temperature of the hot air on the upper surface side during heat shrinkage.
This is because the plastic film located on the lower side of the tray is not required to have a particularly excellent deformation recovery property, and it is easier to obtain a good appearance of the package by heat shrinking as much as possible. The hot air or the like on the lower surface side when the heat shrinks is ± 0 with respect to the temperature of the hot air or the like on the upper surface side when the heat shrinks.
It is preferable to set the temperature to -40 ° C.

The average temperature at which the maximum heat shrinkage stress of the plastic film appears can be measured by the following method.

A plastic film was sampled in a strip shape with a width of 15 mm, and this was set on a chuck with a strain gauge with a chuck width of 50 mm without heating.
A value obtained by dividing the shrinkage force generated by immersing it in silicone oil heated to the measurement temperature by the cross-sectional area of the plastic film before immersion is defined as the shrinkage stress. Then, the temperature at which the maximum shrinkage stress value is obtained by changing the measurement temperature is taken as the temperature at which the maximum shrinkage stress is obtained, and the maximum shrinkage is calculated as the average value of the temperatures at which the maximum shrinkage stress in the longitudinal and transverse directions of the plastic film is obtained. The average temperature at which the stress appears can be obtained.

[0035]

【Example】

Example 1 The surface layer was an ethylene-vinyl acetate copolymer (hereinafter referred to as "EV
A ”), a middle layer of an ethylene-α-olefin copolymer and a central layer of a polypropylene resin having a symmetrical structure of 5 layers of a plastic film are stretched by an inflation method to form a heat-shrinkable plastic film having a thickness of 11 μm. And Table 1 shows the average temperature at which the maximum shrinkage stress of this plastic film appears, the heat shrinkage rate, and the tensile work recovery rate.

Using this plastic film, the tray was wrapped by the pillow packaging method shown in FIG. 2, the inside was evacuated and hermetically sealed. After applying hot air from the lower part of the obtained preliminary packaging to heat-shrink the plastic film on the peripheral side other than the tray lower surface and the tray upper surface, further apply hot air from the upper part of the preliminary packaging to heat the upper plastic film. Wrinkles were removed by shrinking to obtain a package.

The appearance of the obtained package was inspected.
This inspection was visually performed, and the case where the hermetically sealed portion was not on the upper surface of the tray and the plastic film did not have wrinkles, swelling, or swelling was determined to be "good", and any one of them was determined to be "defective".

Also, the deformation recovery of the above-mentioned package was inspected. In this inspection, the surface of a stainless steel rod with a diameter of 15 mm, the surface of which is satin-finished and the tip of which is hemispherical, is applied to the plastic film, and the plastic film is pushed in until the depth becomes 20 mm from the upper surface of the tray. After pulling out and observing the surface condition of the plastic film after 3 minutes, those with almost no dents, slacks or wrinkles are “good”
And the existence of these was defined as “poor”.

The results of the above inspection are shown in Table 1 together with the temperature of hot air.

COMPARATIVE EXAMPLE 1 Except that the atmosphere of the entire periphery of the preliminary package was set as the temperature of the hot air applied from the upper portion in Example 1 without first applying the hot air from the lower portion of the tray to the heat shrink of the plastic film. The packaging was performed in the same manner as in Example 1 and the same inspection was performed.

The results are shown in Table 1.

Comparative Example 2 Packaging was performed in the same manner as in Example 1 except that the hermetic sealing was performed without degassing the inside, and the same inspection was performed.

The results are shown in Table 1.

Example 2 A three-layer plastic film having a symmetrical structure in which the surface layer was ultra-low density polyethylene (VL) and the central layer was linear low-density polyethylene having a higher density than the resin used for the surface layer was prepared by inflation method. The film was stretched to form a heat-shrinkable plastic film having a thickness of 12 μm. Table 1 shows the average temperature at which the maximum shrinkage stress of this plastic film appears, the heat shrinkage rate, and the tensile work recovery rate.

Using the above plastic film, packaging was performed in the same manner as in Example 1 except that the hot air temperature was changed, and the same inspection as in Example 1 was performed.

The results of the inspection are shown in Table 1 together with the temperature of hot air.

Example 3 Inflation was carried out on a five-layer plastic film having a symmetrical structure, in which the surface layer was ultra-low density polyethylene (VL), the central layer was polypropylene resin, and the intermediate layer was a mixture of the surface layer and the resin used for the central layer. Film is drawn by the method and the thickness is 12μ
m of the heat-shrinkable plastic film. Table 1 shows the average temperature at which the maximum shrinkage stress of this plastic film appears, the heat shrinkage rate, and the tensile work recovery rate.

Using the above plastic film, packaging was performed in the same manner as in Example 1 except that the hot air temperature was changed, and the same inspection as in Example 1 was performed.

The results of the inspection are shown in Table 1 together with the temperature of hot air.

Comparative Example 3 A three-layer plastic film having a symmetrical structure in which the surface layer was EVA and the central layer was linear low-density polyethylene was stretched by an inflation method to obtain a heat-shrinkable plastic film having a thickness of 13 μm. Table 1 shows the average temperature at which the maximum shrinkage stress of this plastic film appears, the heat shrinkage rate, and the tensile work recovery rate.

Using the above plastic film, packaging was performed in the same manner as in Example 1 except that the hot air temperature was changed, and the same inspection as in Example 1 was performed.

The results of the inspection are shown in Table 1 together with the temperature of hot air.

Comparative Example 4 A five-layer plastic film having a symmetrical structure comprising EVA, polypropylene resin as the surface layer, and a mixture of the resins used as the intermediate layer for the surface layer and the center layer was stretched by inflation to form a film. It was a heat-shrinkable plastic film of 11 μm. Table 1 shows the average temperature at which the maximum shrinkage stress of this plastic film appears, the heat shrinkage rate, and the tensile work recovery rate.

Using the above plastic film, packaging was performed in the same manner as in Example 1 except that the hot air temperature was changed, and the same inspection as in Example 1 was performed.

The results of the inspection are shown in Table 1 together with the temperature of hot air.

[0056]

[Table 1]

[0057]

The present invention is as described above, and can be sealed without leaving pinholes.
In the case of food packaging, it is easy to maintain hygiene, and there is no deterioration in appearance due to sideways sealing seal parts moving up to the upper surface side or pressing marks, and it is easy to maintain high commercial value and exhibition effect. Is what you get.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a hysteresis curve used for obtaining an average tensile work recovery rate.

FIG. 2 is an explanatory diagram of a pillow packaging system.

FIG. 3 is an explanatory diagram of a half-fold film system.

FIG. 4 is an explanatory diagram of an upper and lower film system.

[Explanation of symbols]

 1 Tray 2 Hermetically sealed part 3 Plastic film

Claims (1)

[Claims] 1. A heat shrinkability in which the heat shrinkage is 20% or more in both the longitudinal and transverse directions, the average value of the heat shrinkage in the longitudinal and lateral directions is 35% or more, and the average tensile work recovery rate after the heat shrinkage is 30% or more. Using a biaxially stretched plastic film, the tray containing the contents is wrapped to deaerate the inside and hermetically seal the opening to form a preliminary package having a hermetically sealed portion on the side of the tray. A shrink wrapping method, wherein the plastic film on the lower surface side of the tray of the preliminary packaging is heat-shrinked to pull the side hermetically sealed portion downward, and then the plastic film on the upper surface side of the tray is thermally shrunk.